Cooling apparatuses and power electronics modules comprising the same

ABSTRACT

Cooling apparatuses and power electronics modules with cooling apparatuses are disclosed. In one embodiment, a cooling apparatus includes a heat transfer plate having a heat output surface and a periodic fractal pattern formed in the heat output surface. The periodic fractal pattern increases the surface area of the heat output surface and provides vapor bubble nucleation sites. An enclosure encloses the heat transfer plate and forms a fluid chamber between the enclosure and the heat transfer plate. A fluid source is fluidly coupled to the fluid chamber and provides cooling fluid to the fluid chamber. When the heat transfer plate is thermally coupled to the heat source, the heat source heats the transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the heat source.

TECHNICAL FIELD

The present specification generally relates to apparatuses for coolingheat generating devices and, more specifically, to cooling apparatusesand power electronics modules utilizing a heat transfer plate with aperiodic fractal pattern on a heat output surface.

BACKGROUND

Power electronics are commonly utilized in a variety of commercial andindustrial applications. In general, the amount of heat generated by apower electronics device increases with increased power output of thedevice. However, such power electronics may cease to function and/ormalfunction when the devices overheat. Accordingly, the power output ofsuch devices may be limited by temperature considerations.

Heat sinks are commonly used in conjunction with heat generatingdevices, such as power electronics devices, to dissipate heat emitted bythe device. The heat sinks are thermally coupled to the heat generatingdevices such that heat from the device is conveyed to the heat sink anddissipated by various mechanisms. In some heat sinks a flow of coolingfluid may be used to receive heat generated by the heat generatingdevice by convective thermal transfer. The flow of cooling fluid carriesthe heat away from the heat generating device. For example, the heatsink may utilize a jet of cooling fluid which is directed such that itimpinges on a surface of the heat generating device. Another method forremoving heat from a heat generating device is to couple the device to afinned heat sink made of a thermally conductive material, such asaluminum. Heat produced in the heat generating device is convectivelytransferred to the heat generating device which, in turn, radiates theheat away from the heat generating device.

However, as the power output of power electronics devices increases tomeet the demands of newly developed electronic systems, conventionalheat sinks may be unable to adequately remove this increased heat fluxto effectively lower the temperature of the power electronics devices toacceptable temperature levels.

Accordingly, a need exists for alternative apparatuses for cooling heatgenerating devices, particularly power electronics devices.

SUMMARY

In one embodiment, a cooling apparatus for a heat source may include aheat transfer plate comprising a heat output surface and a periodicfractal pattern formed in the heat output surface. The periodic fractalpattern increases the surface area of the heat output surface andprovides vapor bubble nucleation sites. An enclosure may enclose atleast the heat output surface of the heat transfer plate such that theenclosure forms a fluid chamber between the enclosure and the heatoutput surface of the heat transfer plate. A fluid source may be fluidlycoupled to the fluid chamber. The fluid source may provide cooling fluidto the fluid chamber. When the heat transfer plate is thermally coupledto the heat source, the heat source heats the transfer plate whichvaporizes the cooling fluid in the fluid chamber thereby dissipating theheat of the heat source.

In another embodiment, a power electronics module may include a heattransfer plate having a heat input surface, a heat output surface, and aperiodic fractal pattern formed in the heat output surface. The periodicfractal pattern increases the surface area of the heat output surfaceand provides vapor bubble nucleation sites. A power electronics devicemay be thermally coupled to the heat input surface of the heat transferplate. An enclosure encloses at least the heat output surface of theheat transfer plate such that the enclosure forms a fluid chamberbetween the enclosure and the heat output surface of the heat transferplate. A vapor condenser may be coupled to the fluid chamber. A fluidsource may be fluidly coupled to the fluid chamber and provide coolingfluid to the fluid chamber. The power electronics device heats the heattransfer plate which vaporizes the cooling fluid in the fluid chamberthereby dissipating the heat of the power electronics device and thevapor condenser condenses cooling fluid vapor in the fluid chamber andreturns the cooling fluid to the fluid source.

In yet another embodiment, a cooling apparatus for a power electronicsmodule includes a heat transfer plate comprising a periodic fractalpattern comprising a plurality of fractal units, each fractal unithaving a maximum depth from about 250 nm to about 500 nm. The periodicfractal pattern increases a surface area of the heat transfer plate andprovides vapor bubble nucleation sites. An enclosure encloses at leastthe heat output surface of the heat transfer plate and forms a fluidchamber between the enclosure and the heat output surface of the heattransfer plate. A vapor condenser may be coupled to the fluid chamber. Afluid source may be fluidly coupled to the fluid chamber such that thefluid source provides cooling fluid to the fluid chamber. The powerelectronics device heats the heat transfer plate which vaporizes thecooling fluid in the fluid chamber thereby dissipating the heat of thepower electronics device and the vapor condenser condenses cooling fluidvapor in the fluid chamber and returns the cooling fluid to the fluidsource.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 schematically depicts a cross section of a cooling apparatusthermally coupled to a heat generating device in a power electronicsmodule, according to one or more embodiments shown and described herein;

FIG. 2 schematically depicts a heat transfer plate of the coolingapparatus of FIG. 1 submerged in a pool of cooling fluid;

FIG. 3A schematically depicts a periodic fractal pattern formed in theheat output surface of a heat transfer plate, according to one or moreembodiments shown and described herein;

FIG. 3B schematically depicts a fractal unit of the periodic fractalpattern of FIG. 3A;

FIG. 4A schematically depicts another embodiment of a periodic fractalpattern formed in the heat output surface of the heat transfer plate;

FIG. 4B schematically depicts a fractal unit of the periodic fractalpattern of FIG. 4A;

FIG. 5 schematically depicts a cross section of a cooling apparatusthermally coupled to a heat generating device in a power electronicsmodule, according to one or more embodiments shown and described herein;

FIG. 6 graphically depicts the boiling curve for a saturated liquid(i.e., a cooling fluid) in terms of the heat flux from a substrate(y-axis) and the change in temperature of the substrate (x-axis); and

FIG. 7 graphically depicts theoretical pool boiling curves for heattransfer plates with different surface finishes.

DETAILED DESCRIPTION

FIG. 1 generally depicts one embodiment of a cooling apparatus for usewith a power electronics module. The cooling apparatus may generallycomprise a heat transfer plate having a periodic fractal pattern formedin a heat output surface, an enclosure, and a fluid source. Theenclosure encloses at least the heat output surface of the heat transferplate and forms a fluid chamber between the enclosure and the heatoutput surface. The fluid source supplies a flow of cooling fluid to thefluid chamber such that the cooling fluid cools the heat output surface.In some embodiments, the cooling apparatus may additionally include avapor condenser coupled to the fluid chamber. Various embodiments of thecooling apparatus and the operation of the cooling apparatus will bedescribed in more detail herein.

Referring to FIG. 1, one embodiment of a cooling apparatus 100 isschematically depicted. The cooling apparatus 100 generally comprises aheat transfer plate 102, an enclosure 104 and a fluid source 108. In theembodiments described herein, the cooling apparatus 100 may alsocomprise a vapor condenser 106. The heat transfer plate 102 is formedfrom a thermally conductive material which facilitates the conduction ofthermal energy through the heat transfer plate from a heat input surface132 to a heat output surface 130. In the embodiment shown in FIG. 1, theheat input surface 132 and the heat output surface 130 are generallyparallel with one another. However, it should be understood that, inother embodiments, the heat input surface 132 and the heat outputsurface 130 may have other relative configurations. Suitable materialsfrom which the heat transfer plate 102 may be formed include, withoutlimitation, copper, aluminum, thermally enhanced composite materials,and polymer composite materials. The heat transfer plate 102 may beformed by a molding process, a machining process or similar processes toachieve the desired shape and configuration.

Referring now to FIGS. 2 and 3A-3B, the heat output surface 130 of theheat transfer plate 102 is formed with a periodic fractal pattern. Oneexemplary embodiment of a portion of a heat output surface 130 a with anexemplary periodic fractal pattern 134 a is schematically depicted inFIG. 3A. The periodic fractal pattern 134 a is formed from a pluralityof fractal units 136 a (FIG. 3B) which are repeated on the heat outputsurface 130 a.

In the embodiments described herein, each fractal unit 136 a generallyhas a width from about 100 nm to about 500 nm and a length from about100 nm to about 500 nm. Further, each fractal unit generally comprises aplurality of fractal sub-units 137 a (one shown as shaded in FIG. 3B)which are interconnected to form the fractal unit 136 a. Accordingly, itshould be understood that each fractal unit 136 a is formed from aplurality of interconnected fractal sub-units 137 a to form a continuousfractal unit 136 a. Moreover, each fractal unit 136 a is repeated on theheat output surface 130 a to form the periodic fractal pattern 134 a. Insome embodiments (not shown) the periodic fractal pattern 134 a isformed by interconnecting individual fractal units 136 a while, in otherembodiments, the periodic fractal pattern is formed by arrangingdiscrete fractal units in a regular pattern as illustrated in FIG. 3A.

As shown in FIGS. 3A and 3B, the periodic fractal pattern 134 a consistsof individual fractal units 136 a which resemble snowflakes or icecrystals. FIGS. 4A and 4B depict another exemplary embodiment of aperiodic fractal pattern 134 b formed in a heat output surface 130 b. Inthis embodiment, the periodic fractal pattern 134 b is also formed froma plurality of fractal units 136 b which resemble snowflakes or icecrystals, as described above. However, while FIGS. 3A-3B and 4A-4 bdepict fractal units that resemble snowflakes or ice crystals, it shouldbe understood that other types of fractal units may also be used.

Referring again to FIGS. 3A and 3B, the periodic fractal patterns 134 adescribed herein may be formed in the heat output surface 130 a to adepth up to about 500 nm. For example, in some embodiments, the depth ofthe periodic fractal pattern 134 a may be from about 250 nm to about 500nm. The periodic fractal pattern 134 a may be formed in the heat outputsurface 130 a by laser ablation, laser etching, ion milling,photo-etching, photolithography or any other technique suitable forforming nano-scale structures in the surface of a component. Forexample, in one embodiment, the periodic fractal pattern 134 a is formedusing a step and flash imprint lithography technique.

In one embodiment, forming the periodic fractal pattern in the surfaceof the heat output surface entails removing material from within theperiodic fractal pattern thereby creating a pocket or depression in theheat output surface. For example, referring to FIG. 3B, the materialwithin each fractal sub-unit 137 a may be removed from the heat outputsurface 130 a such that the periodic fractal pattern is embedded in theheat output surface 130 a.

Alternatively, the periodic fractal pattern formed in the heat transfersurface may be formed by removing material around each fractal unit suchthat the periodic fractal pattern projects from the heat output surface.Accordingly, it should be understood that the phrase “a periodic fractalpattern formed in the heat transfer surface” describes both periodicfractal patterns embedded in the heat surface and periodic fractalpatterns projecting from the heat transfer surface.

Moreover, it should be understood that the periodic fractal pattern mayhave variations in height (i.e., surface topography) within eachindividual fractal unit. For example, discrete portions of each periodicfractal pattern may be at a first height while other portions may be ata second height.

While not wishing to be bound by theory, it is believed that forming theheat output surface 130 a with periodic fractal patterns as describedherein facilitates improved heat transfer between the heat transferplate 102 and cooling fluid which contacts the heat transfer plate 102.For example, the periodic fractal pattern generally increases thesurface area density of the heat output surface of heat transfer platessuch that the interface area between the cooling fluid and the heatoutput surface is greater thereby improving the heat transfer betweenthe cooling fluid and the heat output surface.

Further, it is also believed that the periodic fractal pattern increasesthe density of vapor bubble nucleation sites on the heat output surfaceand, as such, increases the cooling capability of the heat transferplate. In general, it should be understood that the cooling capacity ofthe heat transfer plate and the cooling apparatus incorporating the heattransfer plate generally increase with an increase in the density ofvapor bubble nucleation sites. The periodic fractal pattern may alsoimprove the wettability of the heat output surface.

Moreover, the periodic fractal pattern may enhance boiling of thecooling fluid on the heat output surface of the heat transfer platethereby improving the cooling efficiency of the heat transfer plate andthe cooling apparatus. More particularly, the periodic fractal patternpromotes the formation of vapor bubbles as a result of both theincreased surface area density and vapor bubble nucleation site density.As the vapor bubbles are released from the heat output surface and heatenergy is carried away from the heat transfer plate, the area vacated bythe bubble is filled with cooling fluid having a lower temperature,thereby drawing more heat energy from the heat transfer plate. Combinedwith the increased surface area density and increased number of vaporbubble nucleation sites, this effect may significantly improve thecooling efficiency of the heat transfer plate and an associated coolingapparatus. The aforementioned effect may be further enhanced when theheat transfer plate is used in conjunction with a flowing cooling fluidstream (as opposed to a stationary pool of cooling fluid) as the coolingfluid moving across the heat output surface cools the surface byconvection and delays the onset of nucleate boiling, as will bedescribed further herein.

Referring again to the cooling apparatus 100 depicted in FIG. 1, theheat transfer plate 102 may be enclosed by the enclosure 104. Theenclosure 104 is generally formed from a material capable ofwithstanding high temperatures (i.e., temperatures greater than about60° C.). Suitable materials from which the enclosure 104 may be formedinclude, without limitation, ceramics, metals, polymer materialssuitable for high temperature applications, and/or composite materialssuitable for high temperature applications.

The enclosure 104 is positioned around the heat transfer plate 102 toform a fluid chamber 122 between the enclosure 104 and the heat transferplate 102. In one embodiment, at least the heat output surface 130 ofthe heat transfer plate 102 is enclosed by the enclosure 104, asdepicted in FIG. 1. In this embodiment, the fluid chamber 122 is formedbetween the enclosure 104 and the heat output surface 130 of the heattransfer plate 102 which enables cooling fluid 110 to directly contactthe heat output surface 130 of the heat transfer plate 102.

In the embodiments described herein, the cooling fluid source 108 isfluidly coupled to the fluid chamber 122 and is configured to delivercooling fluid 110 to the fluid chamber 122 to facilitate cooling of theheat transfer plate 102. The cooling fluid source 108 may be anysuitable source for supplying cooling fluid 110 to the fluid chamber122. For example, in embodiments where the cooling apparatus 100 isutilized in a vehicle, the cooling fluid source 108 may be the radiatorsystem of the vehicle and the cooling fluid 110 may be water, radiatorfluid, or any other suitable cooling fluid. In other embodiments, thecooling fluid source 108 may be a stand-alone system which suppliescooling fluid 110 directly to the fluid chamber 122, as depicted inFIG. 1. In this embodiment, the cooling fluid source 108 is coupled tothe fluid chamber 122 with a fluid inlet conduit 118 which extendsthrough the enclosure 104. The fluid inlet conduit 118 is positionedsuch that relatively low temperature cooling fluid 110 enters the fluidchamber 122 and flows over the heat output surface 130 of the heattransfer plate 102, thereby facilitating the transfer of heat from therelatively high temperature heat transfer plate 102 to the relativelylow temperature cooling fluid 110. In the embodiment of the coolingapparatus 100 depicted in FIG. 1, the cooling fluid source 108 isconfigured to supply cooling fluid 110 to the heat output surface 130such that the cooling fluid 110 forms a pool 112 on the heat outputsurface 130 of the heat transfer plate 102.

In the embodiments of the cooling apparatus 100 described herein, thecooling apparatus 100 further comprises a vapor condenser 106. The vaporcondenser 106 receives cooling fluid vapor 114 from the fluid chamber122, condenses the cooling fluid vapor 114, and returns the condensedcooling fluid to the cooling fluid source 108. Accordingly, it should beunderstood that the vapor condenser 106 is coupled to both the fluidchamber 122 and the cooling fluid source 108. In the embodiment shown inFIG. 1, the vapor condenser 106 is positioned within the enclosure 104in the fluid chamber 122. The vapor condenser 106 is coupled to thecooling fluid source 108 with a fluid outlet conduit 120 such thatcondensed cooling fluid 110 flows from the vapor condenser 106 to thecooling fluid source 108 through the fluid outlet conduit 120.

Still referring to FIG. 1, in the embodiments described herein, thecooling apparatus 100 is incorporated in a power electronics module 300to facilitate cooling the power electronics module 300. For example, thepower electronics module 300 may include at least one heat source 200.In this embodiment, the heat source 200 is at least one powerelectronics device 202. The power electronics device may be one or moresemiconductor devices that may include, without limitation, IGBTs,RC-IGBTs, MOSFETs, power MOSFETs, diodes, transistors, and/orcombinations thereof (e.g., power cards). As an example, the powerelectronics device or devices 202 may be used in an electrical system ofa vehicle, such as in hybrid-electric or electric vehicles (e.g., as aninverter system). Such power electronics devices may generatesignificant heat flux when supplying the propulsion power to thevehicle. However, it should be understood that the embodiments of thecooling apparatuses and power electronics modules described herein mayalso be utilized in other applications and are not limited to vehicularapplications.

The power electronics device 202 is thermally coupled to the heat inputsurface 132 of the heat transfer plate 102 to facilitate the transfer ofheat from the power electronics device 202 to the cooling apparatus 100.In one embodiment (not shown), the power electronics device 202 isdirectly coupled to the heat input surface 132 of the heat transferplate 102, such as when the power electronics device 202 is adhesivelybonded directly to the heat transfer plate 102, mechanically coupled tothe heat transfer plate such with mechanical fasteners, or evenproximity coupled (i.e., by surface to surface contact) with the heattransfer plate 102.

However, in alternative embodiments, a thermally conductive substratelayer 204 is positioned between the power electronics device 202 and theheat transfer plate 102, as depicted in FIG. 1. The thermally conductivesubstrate layer 204 assists in transferring the heat generated by thepower electronics device 202 to the cooling apparatus 100. Accordingly,it should be understood that the thermally conductive substrate layer204 is formed from thermally conductive materials. For example, thethermally conductive substrate layer 204 may comprise a direct bondedaluminum substrate, a direct bonded copper substrate, or another similarthermally conductive substrate layer which is directly bonded to theheat transfer plate 102.

The power electronics device 202 may be coupled to the thermallyconductive substrate layer 204 using any appropriate coupling method.For example, in one embodiment, a layer of bonding material (not shown)may be used to couple the power electronics device 202 to the thermallyconductive substrate layer 204 and the cooling apparatus 100. By way ofexample and not limitation, the bond layer may comprise a solder layer,a nano-silver sinter layer, or a transient-liquid-phase bonding layer.Alternatively, the thermally conductive substrate layer 204 may bemechanically coupled to both the heat transfer plate 102 and the powerelectronics device 202 with mechanical fasteners such as screws, bolts,clips or the like.

While the cooling apparatus 100 is shown and described herein as beingincorporated in a power electronics module 300, it should be understoodthat the cooling apparatus 100 may be used in other applications. By wayof example and not limitation, the cooling apparatus 100 may beincorporated in laser devices or in any other device in which thedissipation of thermal energy is needed. The cooling apparatusesdescribed herein are particularly well suited for cooling devices whichgenerate a heat density of 100 W/cm² or more although they are equallysuitable for cooling devices with lower heat densities.

The operation of the cooling apparatus 100 will now be described indetail with specific reference to the cooling apparatus 100 incorporatedin a power electronics module 300 depicted in FIG. 1 and the crosssection of the heat transfer plate 102 depicted in FIG. 2.

Cooling fluid 110 is supplied to the fluid chamber 122 of the coolingapparatus 100 through the inlet conduit 118. In this embodiment, thecooling fluid 110 is supplied to the fluid chamber 122 at a rate whichis sufficient to create a pool 112 of cooling fluid 110 on the heatoutput surface 130 of the heat transfer plate 102. Accordingly, itshould be understood that the cooling fluid 110 is supplied at a ratewhich is greater than the rate at which the cooling fluid 110 isvaporized on the heat output surface 130 of the heat transfer plate 102such that the pool 112 of cooling fluid 110 is created and maintained onthe heat transfer plate 102.

As noted hereinabove, the periodic fractal pattern formed in the heatoutput surface 130 of the heat transfer plate 102 increases thewettability of the heat output surface 130 with respect to the coolingfluid 110 thereby providing a uniform wetting of the heat output surface130 by cooling fluid 110. The periodic fractal pattern increases thesurface area density of the heat output surface 130 thereby providingmore surface area on which cooling fluid vapor bubbles may nucleate.Moreover, the periodic fractal pattern also increases the density ofvapor bubble nucleation sites on the heat output surface 130 therebyproviding a greater number of vapor bubble nucleation sites per unitarea of the heat transfer surface. The periodic fractal pattern alsodisrupts the liquid to vapor boundary thereby further lowering thenucleation energy required for bubble formation.

As the temperature of the power electronics device 202 increases,thermal energy 206 (i.e., heat) generated in the power electronicsdevice 202 flows from the power electronics device 202, through thethermally conductive substrate layer 204 and into the heat input surface132 of the heat transfer plate 102, thereby heating the heat transferplate. As the temperature of the heat transfer plate 102 increases,thermal energy 206 is radiated from the heat output surface 130 and intothe cooling fluid 110, thereby heating the cooling fluid 110. Whensufficient thermal energy is radiated from the heat output surface 130,bubbles 116 of cooling fluid vapor 114 nucleate and grow until thebubble 116 rises to the surface of the pool 112 and the cooling fluidvapor 114 is released into the fluid chamber 122, carrying with it thethermal energy imparted to the cooling apparatus 100 by the powerelectronics device 202.

The cooling fluid vapor 114 rises upward, into the vapor condenser 106where the cooling fluid vapor 114 is condensed back into liquid phasecooling fluid 110 which flows from the condenser unit into the coolingfluid source 108 through the outlet conduit 120. Thereafter, the coolingfluid 110 is re-circulated into the fluid chamber 122 where the processis repeated, such that the thermal energy 206 emitted by the powerelectronics device 202 is continuously removed from the powerelectronics device and the temperature of the power electronics module300 is reduced.

While the embodiment of the cooling apparatus 100 and power electronicsmodule 300 depicted in FIG. 1 transfer heat away from the powerelectronics device 202 by boiling a pool of cooling fluid 110 thatcollects on the heat output surface 130 of the heat transfer plate 102,it should be understood that other configurations are possible. Forexample, in an alternative embodiment, the cooling apparatus may utilizejets of cooling fluid to remove heat from the heat transfer plate, as inthe embodiment of the cooling apparatus 150 depicted in FIG. 5.

Referring to FIG. 5, an alternative embodiment of the cooling apparatus150 is schematically depicted coupled to a heat source 200, specificallya power electronics device 202, in a power electronics module 300. Inthis embodiment, the cooling apparatus 150 comprises a heat transferplate 102, an enclosure 104, a cooling fluid source 108 and a vaporcondenser 106, as described hereinabove with respect to FIG. 1. Also,the heat output surface 130 of the heat transfer plate 102 is formedwith a periodic fractal pattern to improve heat transfer from the heatoutput surface 130 to the cooling fluid 110, as described above.

However, in this embodiment, the cooling fluid 110 is supplied to thefluid chamber 122 as at least one cooling fluid stream 164 emitted froma cooling fluid jet 162 and impinged against the heat output surface 130of the heat transfer plate 102. Specifically, in this embodiment, thecooling fluid 110 is pumped from the cooling fluid source 108 into afluid manifold 160 with a pump (not shown). The manifold 160 comprisesat least one jet 162 (a plurality of jets 162 are schematically depictedin FIG. 5) positioned in the fluid chamber 122 of the cooling apparatus150. In the embodiment shown in FIG. 5, the at least one jet 162 extendsfrom the manifold 160, through the enclosure 104 and the vapor condenser106. The at least one jet 162 is oriented to direct a pressurizedcooling fluid stream 164 onto the heat output surface 130 of the heattransfer plate 102 such that the heat transferred from the powerelectronics device 202 to the heat transfer plate 102 can be dissipatedby vaporization of the cooling fluid. In one embodiment, the at leastone jet 162 is positioned to direct the cooling fluid stream 164 ontothe center of the periodic fractal pattern, such as when the periodicfractal pattern is centered on the heat output surface 130 of the heattransfer plate 102.

In operation, cooling fluid 110 from the cooling fluid source 108 ispumped into the manifold 160 and into the jets 162 from the coolingfluid source 108. The jets 162 pressurize the cooling fluid 110 and emita pressurized cooling fluid stream 164 which is directed onto the heatoutput surface 130 of the heat transfer plate 102. As noted hereinabove,the periodic fractal pattern formed in the heat output surface 130 ofthe heat transfer plate 102 improves the ability of the heat transferplate 102 to transfer heat energy to the cooling fluid 110.

Specifically, as the temperature of the power electronics device 202increases, thermal energy 206 generated in the power electronics device202 flows from the power electronics device 202, through the thermallyconductive substrate layer 204 and into the heat input surface 132 ofthe heat transfer plate 102, thereby heating the heat transfer plate. Asthe temperature of the heat transfer plate 102 increases, thermal energyis radiated from the heat output surface 130. Simultaneously, thepressurized cooling fluid stream 164 from the jets 162 are impingedagainst the heat output surface 130. The cooling fluid wets the heatinput surface 132 assisted by the periodic fractal pattern, as describedabove. As the cooling fluid begins to vaporize, bubbles of cooling fluidnucleate on the heat output surface 130 and grow (i.e., boil) untilsufficient heat energy is available to release the cooling fluid vapor114 into the fluid chamber 122, carrying with it the thermal energyimparted to the cooling apparatus 150 by the power electronics device202.

The cooling fluid vapor 114 rises upward, into the vapor condenser 106where the cooling fluid vapor 114 is condensed back into liquid phasecooling fluid 110 which flows from the condenser unit into the fluidsupply source through the outlet conduit 120. Thereafter, the coolingfluid 110 is re-circulated into the fluid chamber 122 where the processis repeated, such that the thermal energy 206 emitted by the powerelectronics device 202 is continuously removed from the powerelectronics device and the temperature of the power electronics module300 is reduced.

Referring now to FIG. 6, a typical boiling curve 400 for a saturatedfluid, such as a cooling fluid, is graphically depicted as a function ofthe change in temperature of the surface of the heat output surface(ΔT_(sub)). As shown in FIG. 6, the heat flux from the heat outputsurface rapidly increases at the onset 402 of nucleate boiling until amaximum heat flux 404 is reached prior to entering a transition boilingstage. In the transition boiling stage, the boiling of the cooling fluidis unstable and may rapidly progress until the all fluid is boiled fromthe surface thereby terminating cooling. Accordingly, while maximizingthe heat flux from the heat output surface is desirable, it is temperedby the risk of unstable boiling and loss of cooling capacity and, in theworst scenario, overheating.

The cooling apparatuses described herein may assist in improving thecooling capacity of a heat transfer substrate without the necessity ofreaching the maximum heat flux and the corresponding risk of unstableboiling. While not being bound by theory, it is believed that theperiodic fractal pattern provides the heat output surface of the heattransfer plate with an increased surface area density and increasedvapor bubble nucleation site density, as described above. Both of thesefactors may improve the exchange of heat between the heat output surfaceand the cooling fluid. Moreover, the use of a cooling fluid streamimpinged against the heat output surface provides for convective coolingof the heat output surface which, in turn, delays the onset of nucleateboiling while still providing for two-phase heat transfer (i.e., fromliquid phase to vapor phase). In particular, the cooling fluid streamsimpinged against the heat output surface convectively cool the heatoutput surface while, at the same time, cooling vapor bubbles arenucleated on the heat output surface thereby cooling the heat outputsurface by two-phase heat transfer. These two processes used inconjunction delay the onset of nucleate boiling while increasing theheat flux from the surface relative to single phase forced convection asshown in FIG. 6.

Referring to FIG. 7, theoretical boiling curves for heat transfer plateswith a periodic, nano-scale fractal pattern (A), a randomly roughenedsurface (B), and a polished surface (C) are graphically depicted. Whilenot wishing to be bound by theory, it is believed that the heat transferplates with the nano-scale fractal patterns exhibit superior heattransfer characteristics than either a heat transfer plate with arandomly roughened surface or a heat transfer surface with a polishedsurface. Specifically, higher heat fluxes are obtained at lower wallsuperheats (i.e., the temperature of the heat output surface (T_(S))minus the fluid saturation temperature (T_(Sat))) with a heat transfersurface having the periodic nano-scale fractal patterned surfacecompared to either the roughened or polished heat output surfaces. Thismeans that more power may be managed with the periodic nano-scalefractal patterned surface than with either the randomly roughenedsurface or the polished surface for an equivalent temperature gradientacross a heated package.

It should now be understood that the cooling apparatuses describedherein may provide for improved cooling of heat generating devices, suchas power electronics modules and the like. In particular, the heattransfer plates comprising periodic fractal patterns as described hereinmay be used to increase the surface area density and the vapor bubblenucleation site density of the heat transfer plate thereby increasingthe cooling capacity of the heat transfer plate and the coolingapparatus in which the heat transfer plate is utilized.

Moreover, it should be understood that the heat transfer platescomprising periodic fractal patterns may be utilized in pool-boilingconfigurations (e.g., as shown in FIG. 1) or in fluid jet configurations(e.g., as shown in FIG. 5). In either configuration it is believed thatthe cooling apparatuses described herein offer improved coolingcapability over conventional fluid cooling devices.

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue.

While particular embodiments have been illustrated and described herein,it should be understood that various other changes and modifications maybe made without departing from the spirit and scope of the claimedsubject matter. Moreover, although various aspects of the claimedsubject matter have been described herein, such aspects need not beutilized in combination. It is therefore intended that the appendedclaims cover all such changes and modifications that are within thescope of the claimed subject matter.

1. A cooling apparatus for a heat source, the cooling apparatuscomprising: a heat transfer plate comprising a heat output surface and aperiodic fractal pattern formed in the heat output surface, the periodicfractal pattern increasing a surface area density of the heat outputsurface and providing vapor bubble nucleation sites; an enclosureenclosing at least the heat output surface of the heat transfer plate,the enclosure forming a fluid chamber between the enclosure and the heatoutput surface of the heat transfer plate; and a fluid source fluidlycoupled to the fluid chamber, the fluid source providing cooling fluidto the fluid chamber, wherein, when the heat transfer plate is thermallycoupled to the heat source, the heat source heats the transfer platewhich vaporizes the cooling fluid in the fluid chamber therebydissipating the heat of the heat source.
 2. The cooling apparatus ofclaim 1, wherein the periodic fractal pattern comprises a plurality offractal units, each fractal unit having a length from about 100 nm toabout 500 nm and a width from about 100 nm to about 500 nm.
 3. Thecooling apparatus of claim 1, wherein the periodic fractal pattern has adepth less than or equal to about 500 nm.
 4. The cooling apparatus ofclaim 2, wherein each of the plurality of fractal units comprises aplurality of fractal sub-units.
 5. The cooling apparatus of claim 4,wherein the plurality of fractal units are interconnected.
 6. Thecooling apparatus of claim 4, wherein the plurality of fractal units arenot interconnected.
 7. The cooling apparatus of claim 1, furthercomprising a vapor condenser coupled to the fluid chamber and the fluidsource, the vapor condenser condensing cooling fluid vapor in the fluidchamber and returning the cooling fluid to the fluid source.
 8. Thecooling apparatus of claim 1, further comprising a fluid manifoldcoupled to the fluid source, the fluid manifold comprising at least onefluid jet positioned to direct the cooling fluid into the fluid chamber,wherein the at least one fluid jet emits a cooling fluid stream onto theperiodic fractal pattern of the heat transfer plate.
 9. The coolingapparatus of claim 8, wherein the at least one fluid jet is a singlefluid jet and the cooling fluid stream from the single fluid jet impactsthe heat output surface at a center of the periodic fractal pattern. 10.The cooling apparatus of claim 1, wherein the cooling fluid is pooled inthe fluid chamber on the heat output surface of the heat transfer plate.11. The cooling apparatus of claim 1, wherein the heat source is a powerelectronics module coupled to a heat input surface of the heat transferplate.
 12. A power electronics module comprising: a heat transfer platecomprising a heat input surface, a heat output surface, and a periodicfractal pattern formed in the heat output surface, the periodic fractalpattern increasing a surface area density of the heat output surface andproviding vapor bubble nucleation sites; a power electronics devicethermally coupled to the heat input surface of the heat transfer plate;an enclosure enclosing at least the heat output surface of the heattransfer plate, the enclosure forming a fluid chamber between theenclosure and the heat output surface of the heat transfer plate; avapor condenser coupled to the fluid chamber; and a fluid source fluidlycoupled to the fluid chamber, the fluid source providing cooling fluidto the fluid chamber, wherein, the power electronics device heats theheat transfer plate which vaporizes the cooling fluid in the fluidchamber thereby dissipating the heat of the power electronics device andthe vapor condenser condenses cooling fluid vapor in the fluid chamberand returns the cooling fluid to the fluid source.
 13. The powerelectronics module of claim 12, wherein the periodic fractal patterncomprises a plurality of fractal units, each fractal unit having alength from about 100 nm to about 500 nm, a width from about 100 nm toabout 500 nm and a depth from about 250 nm to about 500 nm.
 14. Thepower electronics module of claim 13, wherein the plurality of fractalunits are interconnected.
 15. The power electronics module of claim 13,wherein the plurality of fractal units are not interconnected.
 16. Thepower electronics module of claim 12, further comprising a fluidmanifold coupled to the fluid source, the fluid manifold comprising atleast one fluid jet disposed in the fluid chamber, wherein the at leastone fluid jet emits a cooling fluid stream onto the periodic fractalpattern of the heat transfer plate.
 17. The power electronics module ofclaim 12, wherein the at least one fluid jet is a single fluid jet andthe cooling fluid stream from the single fluid jet impacts the heatoutput surface at a center of the periodic fractal pattern.
 18. Thepower electronics module of claim 12, wherein the cooling fluid ispooled in the fluid chamber on the heat output surface of the heattransfer plate.
 19. A cooling apparatus for a power electronics module,the cooling apparatus comprising: a heat transfer plate comprising aperiodic fractal pattern, the periodic fractal pattern comprising aplurality of fractal units, each fractal unit having a depth less thanor equal to about 500 nm, the periodic fractal pattern increasing asurface area of the heat transfer plate and providing vapor bubblenucleation sites; an enclosure enclosing at least a heat output surfaceof the heat transfer plate, the enclosure forming a fluid chamberbetween the enclosure and the heat output surface of the heat transferplate; a vapor condenser coupled to the fluid chamber; and a fluidsource fluidly coupled to the fluid chamber, the fluid source providingcooling fluid to the fluid chamber, wherein a power electronics deviceof the power electronics module thermally coupled to the heat transferplate heats the heat transfer plate which vaporizes the cooling fluid inthe fluid chamber thereby dissipating the heat of the power electronicsdevice and the vapor condenser condenses cooling fluid vapor in thefluid chamber and returns the cooling fluid to the fluid source.
 20. Thecooling apparatus of claim 19, wherein each fractal unit of theplurality of fractal units has a length from about 100 nm to about 500nm and a width from about 100 nm to about 500 nm.